Explanation about NMR

1. Nuclear Magnetic
Resonance (NMR)
Spectroscopy

2.  Over the past fifty years nuclear magnetic
resonance spectroscopy, commonly referred
to as NMR, has become the preeminent
technique for determining the structure of
organic compounds.
 Of all the spectroscopic methods, it is the
only one for which a complete analysis and
interpretation of the entire spectrum is
normally expected.

3. Background
 The nuclei of many elemental isotopes have a characteristic spin
(I).
 Some nuclei have integral spins (e.g. I = 1, 2, 3 ....)
 Some have fractional spins (e.g. I = 1/2, 3/2, 5/2 ....)
 A few have no spin, I = 0 (e.g. 12C, 16O, 32S, ....)

4. Table of nuclear spin characteristics
Isotope
Natural %
Abundance
Spin (I)
Magnetic
Moment (μ)*
Magnetogyric
/Gyromagnetic Ratio
(γ)†
1H 99.9844 1/2 2.7927 26.753
2H 0.0156 1 0.8574 4,107
11B 81.17 3/2 2.6880 --
13C 1.108 1/2 0.7022 6,728
17O 0.037 5/2 -1.8930 -3,628
19F 100.0 1/2 2.6273 25,179
29Si 4.700 1/2 -0.5555 -5,319
31P 100.0 1/2 1.1305 10,840

5.  Isotopes of particular interest and use to organic chemists are 1H,
13C, 19F and 31P, all of which have I = 1/2.
Since the analysis of this spin state is fairly straightforward, our
discussion of NMR will be limited to these and other I = 1/2
nuclei.

6. NMR phenomenon:
 A spinning charge generates a magnetic field.
The resulting spin-magnet has a magnetic moment
(μ) proportional to the spin.
The frequency of precession is proportional to the strength of the
magnetic field, as noted by the equation: ωo = γBo. The frequency
ωo is called the Larmor frequency and has units of radians per
second. γ is known as the gyromagnetic ratio and is proportional to
the magnetic moment.

7. • In the presence of an external magnetic field (Bo),
two spin states exist, +1/2 and -1/2.
• The magnetic moment of the lower energy +1/2
state is alligned with the external field, but that of
the higher energy -1/2 spin state is opposed to the
external field.

8. Note that the arrow representing the external field points North.
• If rf energy having a frequency matching the
Larmor frequency is introduced at a right angle to
the external field (e.g. along the x-axis), the
precessing nucleus will absorb energy and the
magnetic moment will flip to its I = -1/2 state.

9.  The difference in energy between the two spin
states is dependent on the external magnetic field
strength, and is always very small. The following
diagram illustrates that the two spin states have the
same energy when the external field is zero, but
diverge as the field increases. At a field equal to Bx
a formula for the energy difference is given
(remember I = 1/2 and μ is the magnetic moment
of the nucleus in the field).

10. NMR Spectrophotometer

11.  For spin 1/2 nuclei the energy difference between the two
spin states at a given magnetic field strength will be
proportional to their magnetic moments. For the four
common nuclei noted above, the magnetic moments are:
1H μ = 2.7927, 19F μ = 2.6273, 31P μ = 1.1305 & 13C μ =
0.7022. These moments are in nuclear magnetons, which
are 5.05078•10-27 JT-1.
 The following diagram gives the approximate frequencies
that correspond to the spin state energy separations for
each of these nuclei in an external magnetic field of 2.35
T. The formula in the colored box shows the direct
correlation of frequency (energy difference) with magnetic
moment (h = Planck's constant = 6.626069•10-34 Js).

12. Proton NMR Spectroscopy
 Since protons all have the same magnetic moment, we
might expect all hydrogen atoms to give resonance
signals at the same field / frequency values.
Fortunately this is not true.
 Why should the proton nuclei in different compounds
behave differently in the NMR experiment ?
Since electrons are charged particles, they move in
response to the external magnetic field (Bo) so as to
generate a secondary field that opposes the much stronger
applied field. This secondary field shields the nucleus
from the applied field, so Bo must be increased in order to
achieve resonance. Bo must be increased to compensate
for the induced shielding field. Most organic compounds
exhibit proton resonances that fall within a 12 ppm range,
and it is therefore necessary to use very sensitive and
precise spectrometers to resolve structurally distinct sets
of hydrogen atoms within this narrow range.

13. Chemical Shift

14.  Unlike infrared and uv-visible spectroscopy, where absorption
peaks are uniquely located by a frequency or wavelength, the
location of different NMR resonance signals is dependent on
both the external magnetic field strength and the rf frequency.
Since no two magnets will have exactly the same field,
resonance frequencies will vary accordingly and an alternative
method for characterizing and specifying the location of nmr
signals is needed. This problem is illustrated in the diagram.
Although the eleven resonance signals are distinct and well
separated, an unambiguous numerical locator cannot be
directly assigned to each.
 One method of solving this problem is to report the location of
an nmr signal in a spectrum relative to a reference signal from
a standard compound added to the sample. Such a reference
standard should be chemically unreactive, and easily removed
from the sample after the measurement. Also, it should give a
single sharp nmr signal that does not interfere with the
resonances normally observed for organic compounds.
Tetramethylsilane, (CH3)4Si, usually referred to as TMS,
meets all these characteristics, and has become the reference
compound of choice for proton and carbon nmr.

16.  Since the separation (or dispersion) of NMR signals is magnetic
field dependent, one additional step must be taken in order to
provide an unambiguous location unit. To correct these frequency
differences for their field dependence, we divide them by the
spectrometer frequency (100 or 500 MHz in the example). The
resulting number would be very small, since we are dividing Hz
by MHz, so it is multiplied by a million, as shown by the formula
in the blue shaded box.
 Note that νref is the resonant frequency of the reference signal
and νsamp is the frequency of the sample signal. This operation
gives a locator number called the Chemical Shift, having units of
parts-per-million (ppm), and designated by the symbol δ

19. Proton Chemical Shift Ranges

20.  We can deduce that one factor contributing to chemical shift
differences in proton resonance is the inductive effect. If the
electron density about a proton nucleus is relatively high, the
induced field due to electron motions will be stronger than if the
electron density is relatively low.
 The shielding effect in such high electron density cases will
therefore be larger, and a higher external field (Bo) will be needed for
the rf energy to excite the nuclear spin. Since silicon is less
electronegative than carbon, the electron density about the methyl
hydrogens in tetramethylsilane is expected to be greater than the
electron density about the methyl hydrogens in neopentane (2,2-
dimethylpropane), and the characteristic resonance signal from the
silane derivative does indeed lie at a higher magnetic field. Such
nuclei are said to be shielded.
 Elements that are more electronegative than carbon should exert an
opposite effect (reduce the electron density); and methyl groups
bonded to such elements display lower field signals (they are
deshielded). The deshielding effect of electron withdrawing groups
is roughly proportional to their electronegativity. Furthermore, if more
than one such group is present, the deshielding is additive, and
proton resonance is shifted even further downfield.

21. Proton Chemical Shifts of Methyl Derivatives
Proton Chemical Shifts (ppm)
Compound (CH3)4C (CH3)3N (CH3)2O CH3F
δ 0.9 2.1 3.2 4.1
Compound (CH3)4Si (CH3)3P (CH3)2S CH3Cl
δ 0.0 0.9 2.1 3.0
Compound X=Cl X=Br X=I X=OR X=SR
CH3X 3.0 2.7 2.1 3.1 2.1
CH2X2 5.3 5.0 3.9 4.4 3.7
CHX3 7.3 6.8 4.9 5.0

22. Signal Strength
The magnitude or intensity of NMR resonance signals is
proportional to the molar concentration of the sample. It is
necessary to measure the relative strength as well as the
chemical shift of the resonance signals that comprise an
NMR spectrum. From the relative intensities, together with
the noted chemical shift correlations, the reader should be
able to assign the signals in these spectra to the set of
hydrogens that generates each.
Hint: When evaluating relative signal strengths, it is useful
to set the smallest integration to unity and convert the
other values proportionally.

25. Hydroxyl Proton Exchange
 Experimentally, one simply adds a drop of heavy
water to a chloroform-d solution of the compound and
runs the spectrum again. The result of this exchange
is displayed below.
R-O-H + D2O R-O-D + D-O-H

26. NMR Solvents and Solvent Effects
 Early studies used carbon tetrachloride for this purpose, since
it has no hydrogen that could introduce an interfering signal.
Unfortunately, CCl4 is a poor solvent for many polar
compounds and is also toxic.
 Deuterium labeled compounds, such as deuterium oxide
(D2O), chloroform-d (DCCl3), benzene-d6 (C6D6), acetone-d6
(CD3COCD3) and DMSO-d6 (CD3SOCD3) are now widely
used as NMR solvents. Since the deuterium isotope of
hydrogen has a different magnetic moment and spin, it is
invisible in a spectrometer tuned to protons.
 Because some of these solvents have π-electron functions
and/or may serve as hydrogen bonding partners, the chemical
shifts of different groups of protons may change depending on
the solvent being used.

27. Properties of Some Deuterated NMR Solvents
Solvent B.P. °C
Residual
1H signal (δ)
Residual
13C signal (δ)
acetone-d6 55.5 2.05 ppm 206 & 29.8 ppm
acetonitrile-d3 80.7 1.95 ppm 118 & 1.3 ppm
benzene-d6 79.1 7.16 ppm 128 ppm
chloroform-d 60.9 7.27 ppm 26.4 ppm
cyclohexane-d12 78.0 1.38 ppm 26.4 ppm
dichloromethane-d2 40.0 5.32 ppm 53.8 ppm
dimethylsulfoxide-d6 190 2.50 ppm 39.5 ppm
nitromethane-d3 100 4.33 ppm 62.8 ppm
pyridine-d5 114 7.19, 7.55 & 8.71 ppm 150, 135.5 & 123.5 ppm
tetrahydrofuran-d8 65.0 1.73 & 3.58 ppm 67.4 & 25.2 ppm

28. 1,2-dichloroethane 1,1-dichloroethane
Spin-Spin Interactions

29.  The NMR spectrum of 1,1-dichloroethane (right) is more
complicated than we might have expected from the
previous examples. Unlike its 1,2-dichloro-isomer (left),
which displays a single resonance signal from the four
structurally equivalent hydrogens, the two signals from the
different hydrogens are split into close groupings of two or
more resonances. This is a common feature in the spectra
of compounds having different sets of hydrogen atoms
bonded to adjacent carbon atoms. The signal splitting in
proton spectra is usually small, ranging from fractions of a
Hz to as much as 18 Hz, and is designated as J (referred
to as the coupling constant). In the 1,1-dichloroethane
example all the coupling constants are 6.0 Hz.

30.  The splitting patterns found in various spectra are easily
recognized, provided the chemical shifts of the different sets
of hydrogen that generate the signals differ by two or more
ppm. The patterns are symmetrically distributed on both sides
of the proton chemical shift, and the central lines are always
stronger than the outer lines. The most commonly observed
patterns have been given descriptive names, such as doublet
(two equal intensity signals), triplet (three signals with an
intensity ratio of 1:2:1) and quartet (a set of four signals with
intensities of 1:3:3:1). Four such patterns are displayed in the
following illustration. The line separation is always constant
within a given multiplet, and is called the coupling constant
(J). The magnitude of J, usually given in units of Hz, is
magnetic field independent.

31.  The splitting patterns shown above display the ideal or "First-
Order" arrangement of lines. This is usually observed if the spin-
coupled nuclei have very different chemical shifts (i.e. Δν is large
compared to J). If the coupled nuclei have similar chemical shifts,
the splitting patterns are distorted (second order behavior). In
fact, signal splitting disappears if the chemical shifts are the
same. Two examples that exhibit minor 2nd order distortion are
shown below (both are taken at a frequency of 90 MHz). The
ethyl acetate spectrum on the left displays the typical quartet and
triplet of a substituted ethyl group. The spectrum of 1,3-
dichloropropane on the right demonstrates that equivalent sets of
hydrogens may combine their influence on a second,
symmetrically located set.

33. Even though the chemical shift difference between the A and
B protons in the 1,3-dichloroethane spectrum is fairly large
(140 Hz) compared with the coupling constant (6.2 Hz), some
distortion of the splitting patterns is evident. The line
intensities closest to the chemical shift of the coupled partner
are enhanced. Thus the B set triplet lines closest to A are
increased, and the A quintet lines nearest B are likewise
stronger. A smaller distortion of this kind is visible for the A
and C couplings in the ethyl acetate spectrum.

34.  What causes this signal splitting, and what useful
information can be obtained from it ?
If an atom is perturbed or influenced by a nearby nuclear spin
the observed nucleus responds to such influences, and its
response is manifested in its resonance signal. This spin-
coupling is transmitted through the connecting bonds. When
the perturbing nucleus becomes the observed nucleus, it also
exhibits signal splitting with the same J. For spin-coupling to be
observed, the sets of interacting nuclei must be bonded in
relatively close proximity (e.g. vicinal and geminal locations).
Some spectroscopists place a number before the symbol J to
designate the number of bonds linking the coupled nuclei
(colored orange below). Using this terminology, a vicinal
coupling constant is 3J and a geminal constant is 2J.

35. Magnitude of Some Typical Coupling Constants

36. The following general rules summarize important requirements and
characteristics for spin 1/2 nuclei :
1) Nuclei having the same chemical shift (isochronous) do not exhibit spin-
splitting. They may actually be spin-coupled, but the splitting cannot be observed
directly.
2) Nuclei separated by three or fewer bonds (e.g. vicinal and geminal nuclei) will
usually be spin-coupled and will show mutual spin-splitting of the resonance
signals, provided they have different chemical shifts. Longer-range coupling may
be observed in molecules having rigid configurations of atoms.
3) The magnitude of the observed spin-splitting depends on many factors and is
given by the coupling constant J (units of Hz). J is the same for both partners in a
spin-splitting interaction and is independent of the external magnetic field strength.
4) The splitting pattern of a given nucleus (or set of equivalent nuclei) can be
predicted by the n+1 rule, where n is the number of neighboring spin-coupled
nuclei with the same (or very similar) Js. If there are 2 neighboring, spin-coupled,
nuclei the observed signal is a triplet ( 2+1=3 ); if there are three spin-coupled
neighbors the signal is a quartet ( 3+1=4 ). In all cases the central line(s) of the
splitting pattern are stronger than those on the periphery. The intensity ratio of
these lines is given by the numbers in Pascal's triangle. Thus a doublet has 1:1 or
equal intensities, a triplet has an intensity ratio of 1:2:1, a quartet 1:3:3:1 etc.

39. If a given nucleus is spin-coupled to two or more sets of neighboring
nuclei by different J values, the n+1 rule does not predict the entire
splitting pattern. Instead, the splitting due to one J set is added to that
expected from the other J sets. Bear in mind that there may be fortuitous
coincidence of some lines if a smaller J is a factor of a larger J.

40.  Spin 1/2 nuclei include 1H, 13C, 19F & 31P. The spin-coupling interactions
described above may occur between similar or dissimilar nuclei. If, for
example, a 19F is spin-coupled to a 1H, both nuclei will appear as
doublets having the same J constant. Spin coupling with nuclei having
spin other than 1/2 is more complex and will not be discussed here.

41. Carbon NMR Spectroscopy
 The power and usefulness of 1H NMR spectroscopy as a
tool for structural analysis should be evident from the past
discussion. Unfortunately, when significant portions of a
molecule lack C-H bonds, no information is forthcoming.
Examples:

42.  Even when numerous C-H groups are present,
an unambiguous interpretation of a proton nmr
spectrum may not be possible. The following
diagram depicts three pairs of isomers (A & B)
which display similar proton nmr spectra.

43. Many obstacles needed to be overcome before
carbon NMR emerged as a routine tool :
i) As noted, the abundance of 13C in a sample is
very low (1.1%), so higher sample concentrations
are needed.
ii) The 13C nucleus is over fifty times less sensitive
than a proton in the NMR experiment, adding to the
previous difficulty.
iii) Hydrogen atoms bonded to a 13C atom split its
NMR signal by 130 to 270 Hz, further complicating
the NMR spectrum.

44.  Unlike proton NMR spectroscopy, the relative strength of
carbon NMR signals are not normally proportional to the
number of atoms generating each one. Because of this, the
number of discrete signals and their chemical shifts are the
most important pieces of evidence delivered by a carbon
spectrum.

45. Low Field
Region
High Field
Region
13C Chemical Shift Ranges
* For samples in CDCl3 solution. The δ scale is relative to TMS at δ=0.

46. Examples:

47.  The isomeric pairs previously cited as giving very similar proton
nmr spectra are now seen to be distinguished by carbon nmr. In
the example (blue box), cyclohexane and 2,3-dimethyl-2-butene
both give a single sharp resonance signal in the proton nmr
spectrum (the former at δ 1.43 ppm and the latter at 1.64 ppm).
However, in its carbon nmr spectrum cyclohexane displays a single
signal at δ 27.1 ppm, generated by the equivalent ring carbon
atoms (colored blue); whereas the isomeric alkene shows two
signals, one at δ 20.4 ppm from the methyl carbons (colored
brown), and the other at 123.5 ppm (typical of the green colored
sp2 hybrid carbon atoms).

48.  The C8H10 isomers in the red box have pairs of homotopic
carbons and hydrogens, so symmetry should simplify their NMR
spectra. The fulvene (isomer A) has five structurally different
groups of carbon atoms (colored brown, magenta, orange, blue
and green respectively) and should display five 13C NMR signals
(one near 20 ppm and the other four greater than 100 ppm).
Although ortho-xylene (isomer B) will have a proton NMR very
similar to isomer A, it should only display four 13C NMR signals,
originating from the four different groups of carbon atoms
(colored brown, blue, orange and green). The methyl carbon
signal will appear at high field (near 20 ppm), and the aromatic
ring carbons will all give signals having δ > 100 ppm.

49.  Finally, the last isomeric pair, quinones A & B in the green
box, are easily distinguished by carbon NMR. Isomer A
displays only four carbon NMR signals (δ 15.4, 133.4, 145.8
& 187.9 ppm); whereas, isomer B displays five signals (δ
15.9, 133.3, 145.8, 187.5 & 188.1 ppm), the additional signal
coming from the non-identity of the two carbonyl carbon
atoms (one colored orange and the other magenta).